The two major research aims of our ongoing studies of the effects of high altitude on CNS development and function are to clarify the cellular and molecular mechanisms by which hypoxia impairs brain maturation and to identify CNS-endocrine interrelations during critical stages of brain development. With respect to the first inquiry, work in progress during 1974 has specifically investigated developmental alterations in the protein composition of myelin (particularly specific enzymes basic protein, and lipoproteins) consequent to early exposure to high altitude. Our preliminary findings indicate impaired development of these myelin constitutents and validate our hypothesis that myelinogenesis is affected by hypoxia. Parallel studies on our second inquiry concerned with CNS-endocrine relationships during critical stages of development at high altitude disclose that the alterations in reproductive function observed in adult rats maintained at high altitude may reflect impaired development of hypothalamo-pituitary relationships. Evidence also is accumulating that alterations in gonadotropic levels observed in these animals may be associated with disturbances in CNS monoamine levels and metabolism. Our corollary work directed to assessing hormonal deficiencies in these animals, specifically with respect to thyroxine, suggests that the maturation of the hypothalamo- pituitary-thyroid axis is altered in rats developing at high altitude.